Underground temperatures as indicators of surface temperatures – part 1

Posted by Ari Jokimäki on February 28, 2010

This article was originally written and published by me in Finnish in Ilmastotieto-blog and this is just an English version of it.

Temperatures measured from deep undergound can be used to indicate the surface temperatures in different times. Eventhough the measurements are made with thermometers, they can’t be said to be direct temperature measurements at least for the surface temperature, because there the effects of surface temperature are being measured, not directly the surface temperature. When surface temperature changes, the change is seen immediately in the soil that is in direct contact with the surface. The temperature changes in surface soil are being transferred deeper by heat conduction. Therefore the deeper ground shows the same temperature change as the surface, but in later time. The penetration of a temperature change to the deeper ground takes long time; it takes hundreds of years for a temperature change to show in the depth of few hundred meters. Because of this the deep ground has saved a long time record of Earth’s surface temperature. It just has to be measured from down there. The major source for this article is the review article of Pollack & Huang (2000) of which structure and content is being closely followed in this article. A lot was left unsaid so I urge those interested to read their article.

Historical development

Surface temperature showing in the depths of ground was understood already in early 20th century. At first measurements were taken from mines in order to establish the timings relating to ice ages. Hotchkiss & Ingersoll (1934) write in their abstract:

The retreat of the glacial sheet a number of thousand years ago and the subsequent long-period surface temperature variations must almost certainly have left their impress on the geothermal curve. An attempt has been made to find such an effect and to interpret it in terms of past surface temperatures by subjecting to mathematical analysis a series of geothermal measurements recently made in the Calumet and Hecla conglomerate mine. The results indicate that the glacial epoch ended for this region 20,000-30,000 years ago, and that it was followed by a period with ground temperatures distinctly warmer than the present, succeeded in turn by one cooler, and lasting until comparatively recent times.

Since then also boreholes were being used, as in Beck & Judge (1969):

Heat flow data from a 600-m deep diamond drilled borehole has been used to estimate how short a section of borehole will give a valid heat flow value, to test for recent and ancient climatic changes, underground water-flows and the variation of terrestrial heat flow with depth. Temperatures were repeatedly measured at 3-m intervals; measurements of thermal conductivity, density and porosity were made on specimens sampled at approximately 4-m intervals along the length of the hole. The mean heat flow for the whole borehole before applying any corrections is 0.76 h.f.u. while after correcting for the Wisconsin glaciation the mean value is 1.17 h.f.u., but in both cases some 30 to 100-m sections of the borehole differ by ±20 per cent from the mean values. The differences cannot be entirely explained as being due to structure, topography, climatic changes or underground water-flows.

Cermak (1971) already made a thorough surface temperature reconstruction from the measurements of two boreholes:

There is considerable evidence from different fields of investigation that the world climate has undergone significant variations, even during the last 1,000 years. The effect of the change of temperature on the earth’s surface in the past may be preserved at depths of several hundred feet below the surface. The relation between underground and surface temperature is the reaction of the internal field in a semi-infinite medium to the boundary conditions. Any change at the surface is propagated downwards, and it is shown that the detailed record of temperature with depth can be used to trace the past climatic history. The theory of climatic correction of heat flow is used, and the data is obtained from two boreholes in northeastern Ontario. After analysis the measured underground temperature clearly confirmed the notably warm climate that lasted a few hundred years around A.D. 1000–1200 and the following cold period after 1500.

Both these recent climatic extremes, for which the terms “Little Climatic Optimum” and “Little Ice Age” were coined, are well substantiated, but the magnitude of the temperature variations is uncertain. The relation between mean annual air temperature and surface (ground) temperature depends very much on the precipitation character and the duration of snow cover. The calculated magnitudes of the surface temperature changes probably correspond to the minimum changes of the annual air temperatures, which might have been more pronounced. The results presented indicate for the Kapuskasing area a surface temperature during the Little Climatic Optimum at least 1.5°C higher than the reference value; the mean temperature during the Little Ice Age was about 1°C below this reference value. A remarkable increase since about 1850 reaches value in excess of 3°C.

Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.

There are many methods to measure the subsurface temperatures. One thermometer can be lowered to the borehole and readings are taken from it at different heights. By taking enough readings, the borehole temperature profile can be determined. Another method is to lower a cable with lots of temperature sensors to the borehole. By this method the borehole temperature profile can also be determined. By using this latter method also more can be achieved; by leaving the cable with its many sensors to the borehole for a long time, the changes in the temperature profile can be monitored through time. Nearer surface the sensors can also be buried to the soil. The mines can also be utilized by drilling the sensors deeper into the rock from the walls of the mines. Currently the accuracy of individual measurements is better than one hundreth of a Celsius-degree.

Temperature measurements have been done for thousands of boreholes. However, the data from them is not very well compatible because different methods have been used, measurements have been done with different intervals, and the conditions in measurements sites are sometimes not known well enough. There is however enough of good quality measurements for making temperature reconstructions in many places all over the world. In addition to borehole measurements there is lot of underground temperature data from other fields of study (soil studies, etc.) where measurements have usually been done closer to the surface. Typically the depth of these studies vary from few centimeters to few tens of meters (whereas the depth of boreholes usually is hundreds of meters). The good aspect of these other studies is that usually there’s also lots of other things measured with temperature (surface temperature, soil moisture, etc.) so the conditions of the site are well known.

From the measurements it has been found out that daily variation of surface temperature can be seen in the depth of 2 meters and annual variation of surface temperature can be seen in the depth of 20 meters. Rapid temperature changes are therefore not conveyed very deep so the temperature reconstructions from boreholes don’t show rapid temperature changes, but they show how the temperature has varied during decades and centuries.

There are lot of disturbing factors affecting measurements. Surface topography, vegetation and hydrological conditions affect also to the subsurface temperature. Below surface the temperature profile is being disturbed by changes in groundwater conditions. Historical variations in any of these factors can make the temperature profile to show fallacious climate change. There has been lot of studies on the disturbing factors, but especially in a single measurement site temperature reconstruction the disturing factors can cause a lot of uncertainty.